(1) Technical Field
This invention generally relates to electronic signal switching devices, and more specifically to electronic signal switching devices having high frequency absorptive switch architectures.
(2) Background
Electronic signal switches are used in a wide variety of applications. One type of signal switch in common use is a field effect transistor (FET) that is actively controlled through a gate terminal to block or pass an electrical signal connected in series with the source and drain terminals of the FET. In many applications, the presence of a FET switch has a negligible effect on signals blocked or passed by the switch. However, in radio frequency (RF) circuits, the presence of a FET switch may have significant effects on the rest of the circuit, particularly with respect to termination impedance and isolation levels. Such effects arise because an “ON” (low impedance) FET has a non-zero resistance, and an “OFF” (high impedance) FET behaves as a capacitor.
For example, switch architectures for RF circuits typically have used one or more shunt termination FET switches in series with a fixed termination resistance to form a termination path to circuit ground for unused ports. In most cases, because of the imperfect switching characteristics of a shunt FET switch, an additional FET switch is needed to achieve high isolation of the termination path from the rest of the “OFF path” when the shunt FET is configured as a short to circuit ground.
FIG. 1A is a block diagram of an RF circuit 100 having a shunt switch architecture in accordance with the prior art. All of the switches in the RF circuit 100 may be implemented as FET switches, as described below in further detail. A common RF terminal RFC may be selectively coupled to any of two or more RF pathways corresponding to RF terminals RF1-RFN. For example, each of the RF terminals RF1-RFN may be coupled to respective radio antennas while RFC is coupled to radio transceiver circuitry, such as in a cellular phone. In the illustrated example, to couple terminal RF1 to RFC, a series-coupled RF1 path switch 102 and an isolation switch 104 are set to “ON” (low impedance) to pass signals between RF1 and RFC. Concurrently, one or more associated shunt switches 106 coupled from circuit ground to a signal path 105 between the RF1 path switch 102 and the isolation switch 104 are set to “OFF” (high impedance). In addition, a termination switch 108 connected from circuit ground through a series termination resistor RTerm1 to terminal RF1 is also set to “OFF”.
When terminal RF1 is coupled to RFC as described above, signals pass between RF1 and RFC through the resistance provided by the RF1 path switch 102 and the isolation switch 104, and neither signal path 105 nor RF1 are coupled to circuit ground through the shunt switches 106 or the termination switch 108, respectively. In order to isolate RFC from the other terminals RF2-RFN (all of which have signal paths similar to the RF1 signal path), the other RF pathways are set to be effectively isolated from RFC. When another RF terminal is to be coupled to RFC, then the RF1 signal path must be similarly effectively isolated from RFC. This is accomplished by setting RF1 path switch 102 to “OFF” (high impedance). However, various types of switches—including FET switches—exhibit current leakage when nominally “OFF”. In such embodiments, the shunt switches 106 are provided. Thus, when the RF1 circuit path is to be isolated, the corresponding shunt switches 106 are set to “ON” (low impedance) in order to shunt leakage current through the RF1 path switch 102 to ground.
In order to provide a fixed termination impedance when RF1 is “OFF”, the termination switch 108 is set to “ON” (low impedance) in order to couple RF1 though RTerm1 to circuit ground. The value of RTerm1 is application dependent, but is generally set such that, taking into account the “ON” resistance (Ron) of the termination switch 108 itself, the characteristic impedance at RF1 is about 50 ohms for most RF applications. Without such termination, a nominally “OFF” antenna may reflect received power back into the RF circuit 100 and cause signal interference with other RF circuitry, such as another antenna that is “ON”.
In order to achieve high isolation from RF1 to RFC, and to raise the level of impedance that is effectively in parallel with RTerm1 and termination switch 108, the isolation switch 104 is set to “OFF” (high impedance). In a typical application, the “OFF” resistance of the isolation switch 104 is set at thousands of ohms in order to achieve 20-30 dB of isolation.
As noted above, all of the switches in the RF circuit 100 of FIG. 1A may be implemented as FET switches. FIG. 1B is a schematic diagram of a typical switch configuration 120 suitable for use in the circuit shown in FIG. 1A. Shown are one or more FETs series stacked (for voltage handling) in a conventional configuration. A drain D to source S resistance R of high value ensures that each switch provides uniform resistance when the FETs are set to “OFF” (high impedance) under the control of a signal to the gate structures G; the value of R is application dependent. When the FETs are set to “ON” (low impedance) under the control of a signal to the gate structures G, the resistance Ron from the drain D and source S is quite low but not zero.
A problem with the shunt switch architecture shown in FIG. 1A is that, as the frequency of operation of the RF circuit 100 increases, the combined parasitic FET capacitance of the isolation switch 104 and termination switch 108 respectively begin to degrade both the isolation level and the termination impedance of the RF circuit 100 as a whole. To counter this behavior, the FET termination switch 108 is often made smaller to minimize its capacitance. This in turn means that more of the RF power from the nominally isolated RF1-RFN terminals is terminated in the corresponding termination switches 108 rather than in the corresponding termination resistors RTerm1-RTermN because of the higher resistance of the smaller FET switch devices. Terminating such power in the smaller FET switches can lead to premature failure; to avoid that issue, the power handling capability for the RF circuit 100 would have to be specified at a reduced level, which may be commercially disadvantageous.
Accordingly, there is a need for a switch architecture suitable for use with high frequency RF signals that does not exhibit the problems of the prior art. The present invention meets this need.